In early 2005, ocean surface and subsurface temperatures began to
cool in the equatorial Pacific basin. As a result, the monthly SST
anomalies in the Niño 3.4 region exceeded +0.5°C only
during April of this year, after which the 2004-2005 ENSO warm
event dissipated. At the end of September 2005, the 3-month running
mean of the Niño 3.4 Index continued to decline sharply
and was was approaching zero. (NOTE: A running 3-month mean SST
anomaly above +0.5°C in the Niño 3.4 region is one
indicator that an El Niño is occurring. For the official
NOAA classification scheme, please see NOAA's El
Niño/La Niña Index Definition and see the
CPC ENSO Diagnostic Discussion for NOAA's latest official
assessment of ENSO conditions.)

Equatorial
Zonal Winds (U-Component Winds) and Sea-Level
Topography:The
easterly trade winds were above-normal during September across
the eastern and central tropical Pacific basin, which helped to
enhance equatorial upwelling in the mixed-layer. In the western
equatorial Pacific basin, westerly wind anomalies developed during
the latter half of the month and these moved east past the dateline
by mid-September (see the September
zonal wind loop).

Satellite altimetry of ocean surface topography from the NASA/JPL
Jason-1 satellite over the Pacific basin and global oceans is
shown to the left. The overpass of
the Jason-1 satellite on September 17th showed that increased
sea level associated with tropical convection developed near the
equator in the western Pacific near the end of the month. This was
the first significant sea level rise since late April, but it
remains uncertain whether this event generated an oceanic Kelvin
wave and whether it will persist and move further eastward.

The monthly
averaged OLR Index remained postive in September, with a mean
value of +0.1 averaged across an area centered over the dateline in
the western Pacific (between 160° E and 160° W). The OLR
Index has shifted sign several times over the past year, with no
persistent trend in the index observed so far in 2005. Note that
high frequency variability in OLR is typically associated with the
Madden-Julian
Oscillation (MJO) (MJO related convective activity propagates
west to east in the near-equatorial region from the Indian Ocean
into the Pacific Ocean approximately every 30-60 days). The latest
MJO activity can be seen in CPC's graphs of
Daily MJO Indices.

Southern
Oscillation Index (SOI):
The standardized SOI remained
near-neutral in September, despite a slight increase in the
monthly averaged index to a value of +0.4. The SOI has been
negative during 5 months so far in 2005, with an extraordinarily
low value of -4.1 occurring back in February 2005 (which was the
lowest SOI value since the peak of the 1982-1983 El Niño
event). However, since March the SOI has been either neutral or
near-neutral, which reflects the lack of any large-scale
fluctuations in mean sea-level pressure (MSLP) across the tropical
Pacific basin.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: El Niño/Southern Oscillation for September 2005, published online October 2005, retrieved on March 3, 2015 from http://www.ncdc.noaa.gov/sotc/enso/2005/9.